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Microbial rhodopsins are photoswitchable seven-transmembrane proteins that are widely distributed in three domains of life, archaea, bacteria and eukarya. Rhodopsins allow the transport of protons outwardly across the membrane and are indispensable for light-energy conversion in microorganisms. Archaeal and bacterial proton pump rhodopsins have been characterized using an  Escherichia coli  expression system because that enables the rapid production of large amounts of recombinant proteins, whereas no success has been reported for eukaryotic rhodopsins. Here, we report a phylogenetically distinct eukaryotic rhodopsin from the dinoflagellate  Oxyrrhis marina  ( O. marina  rhodopsin-2,  Om R2) that can be expressed in  E. coli  cells.  E. coli  cells harboring the  Om R2 gene showed an outward proton-pumping activity, indicating its functional expression. Spectroscopic characterization of the purified  Om R2 protein revealed several features as follows: (1) an absorption maximum at 533 nm with all- trans  retinal chromophore, (2) the possession of the deprotonated counterion (p K  a  = 3.0) of the protonated Schiff base and (3) a rapid photocycle through several distinct photointermediates. Those features are similar to those of known eukaryotic proton pump rhodopsins. Our successful characterization of  Om R2 expressed in  E. coli  cells could build a basis for understanding and utilizing eukaryotic rhodopsins.

Functional expression of the eukaryotic proton pump rhodopsin OmR2 in Escherichia coli and its photochemical characterization